[functions] Emulator integration and reloading

[dackers86]

Implement Firebase Emulator Suite integration and hot reloading support for Dart functions. This includes:

• Integration with Firebase Emulator Suite for local development
• Hot reloading logic that automatically reloads the server when user code is modified
• The Firebase CLI triggering reload when the specification file is modified by build_runner

Per the SOW: "The functions SDK will include hot reloading logic, reloading the server automatically whenever user code is modified."

Local Development:
To test locally, use npm link in the firebase-tools repository:

1. Clone firebase-tools repository
2. Run npm install in firebase-tools
3. Run npm link to create global symlink
4. In your test project, run npm link firebase-tools

Dependencies:

• [functions] Dart runtime delegate
• [functions] build_runner integration

Acceptance Criteria:

• Emulator Suite integration working
• Hot reloading on code changes functional
• Reload triggered on spec file changes
• Local development instructions documented